System and method for controlling delivery of ablation energy to tissue

ABSTRACT

A system for controlling delivery of ablation energy by an ablation catheter to tissue in a body is provided. The system includes an electronic control unit configured to determine, responsive to a measurement signal from the ablation catheter, a value for a characteristic associated with the delivery of ablation energy to the tissue. In one embodiment, the characteristic is the degree of contact between the ablation catheter and the tissue. The unit is further configured to generate a control signal, responsive to the determined value of the characteristic, to control an amount of energy delivered from an ablation delivery element on the ablation catheter to the tissue. The amount of energy varies in response to the determined value of the characteristic when the determined value of the characteristic meets a predetermined condition relative to a threshold value for the characteristic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/642,979 filed May 4, 2012, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates to a system and method for controlling deliveryof ablation energy by an ablation catheter to tissue in a body. Inparticular, the instant invention relates to a system and method thatenable fine control or titration of the delivery of ablation energy totissue in response to changes in various characteristics associated withthe delivery of ablation energy to the tissue such as the degree ofcontact between the ablation catheter and tissue.

b. Background Art

It is well known to use ablation catheters to create tissue necrosis incardiac tissue to correct cardiac arrhythmias (including, but notlimited to, atrial fibrillation, atrial flutter, atrial tachycardia andventricular tachycardia). Arrhythmia can create a variety of dangerousconditions including irregular heart rates, loss of synchronousatrioventricular contractions and stasis of blood flow which can lead toa variety of ailments and even death. It is believed that the primarycause of many arrhythmias is stray electrical signals within one or moreheart chambers. The ablation catheter imparts ablative energy (e.g.,radiofrequency energy, light energy, ultrasound, or thermal (cryo orheat based) energy) to the heart tissue to create a lesion in the hearttissue. This lesion disrupts undesirable electrical pathways and therebylimits or prevents stray electrical signals that lead to arrhythmias.

Ablation therapy provides significant benefits in resolving cardiacarrhythmias. There are, however, a number of risks associated withablation therapy. The use of certain types of ablative energy (e.g.radiofrequency energy) generates heat that can lead to formation of athrombus at or near the distal tip of the catheter. If the thrombusbecomes dislodged, there is a risk that the thrombus will travel to alocation where the thrombus will prevent further bloodflow and result instroke. Many catheters employ saline irrigation to cool the tip of thecatheter, but irrigation may fail to sufficiently reduce heat andthrombus formation. Thrombii may also be removed by employing vacuumforce through the sheath in which the catheter is disposed or byremoving the catheter itself, but both of these actions may result indislodgement of the thrombus.

The foregoing discussion is intended only to illustrate the presentfield and should not be taken as a disavowal of claim scope.

BRIEF SUMMARY OF THE INVENTION

Among other things, various embodiments are directed to a system andmethod for controlling delivery of ablation energy by an ablationcatheter to tissue in a body. In particular, system(s) and method(s) aredisclosed that improve control of the delivery of ablation energy tominimize delivery of energy into the blood and reduce the incidence ofthrombus formation during a procedure.

A system for controlling delivery of ablation energy from an ablationcatheter to tissue in a body in accordance with one embodiment of theinvention may include an electronic control unit configured todetermine, responsive to a measurement signal from the ablationcatheter, a value for a characteristic associated with the delivery ofablation energy to the tissue. The electronic control unit is furtherconfigured to generate a first control signal, responsive to thedetermined value of the characteristic, to control an amount of energydelivered from the ablation delivery element to the tissue. The amountof energy varies in response to the determined value of thecharacteristic when the determined value of the characteristic meets apredetermined condition relative to a first threshold value for thecharacteristic. In accordance another embodiment of the invention, thesystem may further include a radio frequency generator configured togenerate an ablation signal responsive to the first control signal. Theablation signal controls the amount of energy delivered from an ablationdelivery element on the ablation catheter to the tissue. In accordancewith yet another embodiment of the invention, the system may furtherinclude a remote catheter guidance system including a manipulatorassembly and a drive assembly supported on the manipulator assembly andcoupled to at least one steering wire of the ablation catheter. Theelectronic control unit may be further configured to generate a secondcontrol signal, responsive to the determined value of thecharacteristic, to control the drive assembly and movement of a distalend of the ablation catheter.

A system for controlling delivery of ablation energy from an ablationcatheter to tissue in a body in accordance with another embodiment ofthe invention includes a computer readable storage medium having acomputer program encoded thereon that when executed by an electroniccontrol unit controls delivery of ablation energy from an ablationcatheter to tissue in a body. The computer program includes code fordetermining, responsive to a measurement signal from the ablationcatheter, a value for a characteristic associated with the delivery ofablation energy to the tissue. The computer program further includescode for generating a first control signal, responsive to the determinedvalue of the characteristic, to control an amount of energy deliveredfrom the ablation delivery element to the tissue. The amount of energyvaries in response to the determined value for the characteristic whenthe determined value of the characteristic meets a predeterminedcondition relative to a first threshold value for the characteristic.

A system and method in accordance with the present invention isadvantageous because the system and method enable finer control of thedelivery of ablation energy to tissue in response to changes in variouscharacteristics associated with the delivery of ablation energy to thetissue. As a result, delivery of ablation energy into the blood isminimized to reduce the risks of thrombus formation.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic view of one embodiment of a system forcontrolling delivery of ablation energy from an ablation catheter totissue in a body in accordance with the present teachings.

FIG. 2 is a diagrammatic view of a remote catheter guidance system foruse in the system of FIG. 1.

FIG. 3 is a flow chart diagram illustrating one embodiment of a methodfor controlling delivery of ablation energy from an ablation catheter totissue in a body in accordance with the present teachings.

FIG. 4 is a plot diagram illustrating a function correlating a change inthe amount of ablation energy generated in response to a change in thedegree of contact between the ablation catheter and tissue in accordancewith one embodiment of the present teachings.

FIG. 5 is a diagrammatic view of a graphical user interface inaccordance with one embodiment of the present teachings through which aclinician may establish a control function correlating a change in theamount of ablation energy generated in response to a change in the valueof a characteristic.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Various embodiments are described herein of various apparatuses,systems, and/or methods. Numerous specific details are set forth toprovide a thorough understanding of the overall structure, function,manufacture, and use of the embodiments as described in thespecification and illustrated in the accompanying drawings. It will beunderstood by those skilled in the art, however, that the embodimentsmay be practiced without such specific details. In other instances,well-known operations, components, and elements have not been describedin detail so as not to obscure the embodiments described in thespecification. Those of ordinary skill in the art will understand thatthe embodiments described and illustrated herein are non-limitingexamples, and thus it can be appreciated that the specific structuraland functional details disclosed herein may be representative and do notnecessarily limit the scope of the embodiments, the scope of which isdefined solely by the appended claims.

Reference throughout the specification to “various embodiments,” “someembodiments,” “one embodiment,” or “an embodiment”, or the like, meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment.Thus, appearances of the phrases “in various embodiments,” “in someembodiments,” “in one embodiment,” or “in an embodiment”, or the like,in places throughout the specification are not necessarily all referringto the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments. Thus, the particular features, structures, orcharacteristics illustrated or described in connection with oneembodiment may be combined, in whole or in part, with the featuresstructures, or characteristics of one or more other embodiments withoutlimitation given that such combination is not illogical ornon-functional.

It will be appreciated that the terms “proximal” and “distal” may beused throughout the specification with reference to a clinicianmanipulating one end of an instrument used to treat a patient. The term“proximal” refers to the portion of the instrument closest to theclinician and the term “distal” refers to the portion located furthestfrom the clinician. It will be further appreciated that for concisenessand clarity, spatial terms such as “vertical,” “horizontal,” “up,” and“down” may be used herein with respect to the illustrated embodiments.However, surgical instruments may be used in many orientations andpositions, and these terms are not intended to be limiting and absolute.

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 1illustrates one embodiment of a system 10 for controlling delivery ofablation energy to tissue 12 in a body 14. In the illustratedembodiment, tissue 14 comprises cardiac tissue within a human body. Itshould be understood, however, that a system 10 in accordance with thepresent teachings may find application in connection with procedures forthe diagnosis or treatment of a variety of tissues in human and on humanbodies. System 10 may include an ablation catheter 16, an ablationgenerator 18, a medical device position and navigation system 20, aremote catheter guidance system (RCGS) 22, a display system 24, and anelectronic control unit (ECU) 26.

Catheter 16 is provided for examination, diagnosis and treatment ofinternal body tissues such as tissue 12. In accordance with oneembodiment of the invention, catheter 16 comprises an ablation catheterand, more particularly, an irrigated radio-frequency (RF) ablationcatheter. It should be understood, however, that catheter 16 is providedfor illustration only and that system 10 could be adapted for use withother types of ablation catheters including those providing differenttypes of ablation energy (e.g., cryoablation, ultrasound, etc.).Catheter 16 is connected to a fluid source 28 having a biocompatiblefluid such as saline through a pump 30 (which may comprise, for example,a fixed rate roller pump or variable volume syringe pump with a gravityfeed supply from fluid source 28 as shown) for irrigation. Catheter 16is also electrically connected to ablation generator 18 for delivery ofRF energy. Catheter 16 may include a cable connector or interface 32, ahandle 34, a shaft 36 having a proximal end 38 and a distal end 40 (asused herein, “proximal” refers to a direction toward the end of thecatheter near the clinician, and “distal” refers to a direction awayfrom the clinician and (generally) inside the body of a patient) and oneor more electrodes 42, 44. Catheter 16 may also include otherconventional components not illustrated herein such as a temperaturesensor, additional electrodes, and corresponding conductors or leads.

Connector 32 provides mechanical, fluid and electrical connection(s) forcables extending from ablation generator 18, RCGS 22, and pump 30.Connector 32 is conventional in the art and is disposed at a proximalend of catheter 16.

Handle 34 provides a location for the clinician to hold catheter 16 andmay further provides means for steering or guiding shaft 36 within body14. For example, handle 34 may include means to change the length of asteering wire extending through catheter 16 to distal end 40 of shaft 36to steer distal end 40 and, thus, shaft 36. Handle 34 is alsoconventional in the art and it will be understood that the constructionof handle 34 may vary and may be absent in a fully-roboticimplementation of the system.

Shaft 36 is an elongated, flexible member configured for movement withinbody 14. Shaft 36 supports electrodes 42, 44, associated conductors, andpossibly additional electronics used for signal processing orconditioning. Shaft 36 may also permit transport, delivery, and/orremoval of fluids (including irrigation fluids and bodily fluids),medicines, and/or surgical tools or instruments. Shaft 36 may be madefrom conventional materials such as polyurethane and defines one or morelumens configured to house and/or transport electrical conductors,fluids, or surgical tools. Shaft 36 may be introduced into a bloodvessel or other structure within body 14 through a conventionalintroducer sheath. Shaft 36 may then be steered or guided through body14 to a desired location such as tissue 12 using RCGS 22 or with guidewires or with pullwires or other means known in the art.

Electrodes 42, 44 are provided for a variety of diagnostic andtherapeutic purposes including, for example, electrophysiologicalstudies, catheter identification and location, pacing, and cardiacmapping and ablation. In the illustrated embodiment, catheter 16includes an ablation tip electrode 42 at distal end 40 of shaft 36 thatfunctions as an ablation delivery element and one or more ringelectrodes 44 that may be used to obtain electrograms for tissue 12 andfor other conventional purposes. It should be understood, however, thatthe number, orientation, and purpose of electrodes 42, 44 may vary.Additional details regarding a catheter, such as catheter 16, may befound in U.S. Pat. No. 7,857,810, the entire disclosure of which isincorporated herein by reference.

Ablation generator 18 generates, delivers and controls radiofrequencyenergy used by catheter 16. Generator 18 a radiofrequency generator 46configured to generate an ablation signal that is output across a pairof source connectors: a positive polarity connector which may connect toelectrode 42 on catheter 16; and a negative polarity connector which maybe electrically connected by conductors or lead wires to a patchelectrode (not shown) on body 14. It should be understood that the termconnectors as used herein does not imply a particular type of physicalinterface mechanism, but is rather broadly contemplated to represent oneor more electrical nodes. Generator 46 is configured to generate asignal at a predetermined frequency in accordance with one or more userspecified parameters (e.g., power, time, etc.) and under the control ofvarious feedback sensing and control circuitry as is know in the art.Generator 46 may generate a signal, for example, with a frequency ofabout 450 kHz or greater. Ablation generator 18 may also monitor variousparameters associated with the ablation procedure including impedance,the temperature at the tip of catheter 16, ablation energy and theposition of the catheter 16 and provide feedback to the physicianregarding these parameters.

System 20 is provided to determine the position and orientation ofcatheter 16 and similar devices within body 14. System 20 may comprisethe system offered for sale under the trademark EnSite™ NavX™ by St.Jude Medical, Inc. and described in U.S. Pat. No. 7,263,397, the entiredisclosure of which is incorporated herein by reference. The system isbased on the principle that when low amplitude electrical signals arepassed through the thorax, body 14 acts as a voltage divider (orpotentiometer or rheostat) such that the electrical potential or fieldstrength measured at an electrode on catheter 16 may be used todetermine the position of the electrode, and therefore catheter 16relative to a pair of external patch electrodes using Ohm's law and therelative location of a reference electrode (e.g. in the coronary sinus).In one configuration, the system includes three pairs of patchelectrodes that are placed on opposed surfaces of the body (e.g., chestand back, left and right sides of the thorax, and neck and leg) and formgenerally orthogonal x, y, and z axes as well as a referenceelectrode/patch that is typically placed near the stomach and provides areference value and acts as the origin of the coordinate system for thenavigation system. Sinusoidal currents are driven through each pair ofpatch electrodes and voltage measurements for one or more positionsensors (e.g., electrodes) associated with the medical device areobtained. The measured voltages are a function of the distance of theposition sensors from the patch electrodes. The measured voltages arecompared to the potential at the reference electrode and a position ofthe position sensors within the coordinate system of the navigationsystem is determined. In accordance with this exemplary system, system20 may include patch electrodes 48 (namely 48 _(X1), 48 _(X2), 48 _(Y1),48 _(Y2), 48 _(Z1), 48 _(Z2)) a switch 50, and a signal generator 52.

Patch electrodes 48 are provided to generate electrical signals used indetermining the position of catheter 16 within a three dimensionalcoordinate system 54 of system 20. Electrodes 48 may also be used togenerate EP data regarding tissue 12. Electrodes 48 are placedorthogonally on the surface of body 14 and are used to create axesspecific electric fields within body 14. Electrodes 48 _(X1), 48 _(X2)may be placed along a first (x) axis. Similarly, electrodes 48 _(Y1), 48_(Y2) may be placed along a second (y) axis, and electrodes 48 _(Z1), 48_(Z2) may be placed along a third (z) axis. Each of the electrodes 48may be coupled to multiplex switch 50. ECU 26 is configured throughappropriate software to provide control signals to switch 50 and therebysequentially couple pairs of electrodes 48 to signal generator 52.Excitation of each pair of electrodes 48 generates an electromagneticfield within body 14 and within an area of interest such as the heart.Voltage levels at non-excited electrodes 48 may be filtered andconverted and provided to ECU 26 for use as reference values.

In an alternative embodiment, system 20 may comprise a system thatemploys magnetic fields to detect the position of catheter 16 withinbody 14 such as the MediGuide™ Technology offered for sale by St. JudeMedical, Inc. and generally shown and described in, for example, U.S.Pat. No. 7,386,339, the entire disclosure of which is incorporatedherein by reference. In such a system, a magnetic field generator may beemployed having three orthogonally arranged coils, arranged to create amagnetic field within body 14 and to control the strength, orientation,and frequency of the field. The magnetic field generator may be locatedabove or below the patient (e.g., under a patient table) or in anotherappropriate location. Magnetic fields are generated by the coils andcurrent or voltage measurements for one or more position sensors (e.g.,a coil) associated with catheter 16 are obtained. The measured currentsor voltages are proportional to the distance of the sensors from thecoils thereby allowing a position of the sensors within the coordinatesystem 54 of system 20.

RCGS 22 is provided to manipulate catheter 16. In particular, RCGS 22permits control of translation, distal bending, and virtual rotation ofcatheter 16 and any surrounding sheath. RCGS 22 therefore provides theuser with a type of control similar to that provided by conventionalmanually-operated systems, but allows for repeatable, precise, anddynamic movements. A clinician may identify target locations(potentially forming a path) on an image of tissue 12. RCGS 22 relatesthese digitally selected points to positions within the patient'sactual/physical anatomy, and may thereafter command control the movementof catheter 16 to the defined positions where the clinician or the RCGS22 can perform the desired diagnostic of therapeutic function. Referringto FIG. 2, RCGS 22 may include an input control system 56, an electroniccontrol system 58, and a manipulator assembly 60 for operating atranslation and/or deflection drive assembly or device cartridge 62.

Input control system 56 is provided to allow the clinician to interactwith the RCGS 22 to control movement of catheter 16. System 56 mayinclude, for example, instrumented traditional catheter handle controls,oversized catheter models, instrumented user-wearable gloves, touchscreen display monitors, 2-D input devices, 3-D input devices, spatiallydetected styluses, and traditional joysticks. These input devices may beconfigured to directly control the movement of catheter 16 and anysurrounding sheath, or may be configured, for example, to manipulate atarget or cursor on an associated display.

Electronic control system 58 is configured to translate (i.e.,interpret) inputs (e.g., motions) of the user at an input device or fromanother source into a resulting movement of catheter 16. The electroniccontrol system 58 issues commands to manipulator assembly 60 (i.e., tothe actuation units—electric motors) to move or bend catheter 16 toprescribed positions and/or in prescribed ways, all in accordance withthe received user input and a predetermined programmed operatingstrategy. System 58 may include one or more stand-alone microprocessorsor application specific integrated circuits (ASICs). Alternatively,system 58 may form a part of ECU 26.

Manipulator assembly 60 is configured to maneuver catheter 16 inresponse to commands from electronic control system 58. Assembly 60 maycause translational movement such as advancement or withdrawal ofcatheter 16 and effect deflection of distal end 40 of catheter 16 and/orrotation or virtual motion. Assembly 60 may include conventionalactuation mechanisms (e.g., a plurality of electric motor and lead screwcombinations) for linearly actuating one or more control members (e.g.,steering wires) associated with catheter 16 for achieving theabove-described translation, deflection, and/or rotation (or virtualrotation).

Drive assembly or device cartridge 62 is provided to translate movementof elements in manipulator assembly 60 to catheter 16. Cartridge 62receives and retains proximal end 38 of catheter 16. Cartridge 62 mayinclude sliding blocks 64 each coupled to a corresponding steering wire66 so as to permit independent tensioning of each wire 66. Movement ofthe blocks 64 is controlled by manipulator assembly 60 to causetensioning of the wires 66 and thereby affect translation, deflection,and rotation of catheter 16.

A more complete description of various elements of an RCGS may be foundin the following patent applications that are incorporated herein byreference in their respective entireties: International PatentApplication Publication No. WO 2009/120982 published Oct. 1, 2009; U.S.Patent Application Publication No. 2009/0247942 published Oct. 1, 2009;U.S. Patent Application Publication No. 2009/0247944 published Oct. 1,2009; U.S. Patent Application Publication No. 2009/0247993 publishedOct. 1, 2009; U.S. Patent Application Publication No. 2009/0248042published Oct. 1, 2009; U.S. Patent Application Publication No.2010/0256558 published Oct. 7, 2010; and U.S. Patent ApplicationPublication No. 2011/0015569 published Jan. 20, 2011. Further, althougha particular embodiment of an RCGS 22 has been described and illustratedherein, it should be understood that RCGS 22 may assume a variety ofdifferent embodiments. For example, RCGS 22 may comprise any of thesystems offered for sale by Hansen Medical, Inc. under the trademarksMagellan and Sensei. RCGS 22 may also comprise a magnetic navigationsystem such as the system offered for sale by Stereotaxis, Inc. underthe trademark Epoch in which magnetic fields are used to guide anablation catheter having a magnetic member that is responsive to thegeneration of the magnetic fields.

Display system 24 is provided to convey information to a clinician toassist in diagnosis and treatment. Display system 24 may comprise one ormore conventional computer monitors or other display devices. Displaysystem 24 presents a graphical user interface (GUI) to the clinician.The GUI may include a variety of information including, for example, animage of the geometry of tissue 12, electrophysiology data associatedwith the tissue 12, graphs illustrating voltage levels over time forvarious electrodes 42, 44, and images of catheter 16 and other medicaldevices and related information indicative of the position of catheter16 and other devices relative to the tissue 12.

ECU 26 provides a means for controlling delivery of ablation energy byablation catheter 16 to tissue 12 and for controlling the operation ofvarious components of system 10 including catheter 16, ablationgenerator 18, RCGS 22, display system 24 and switch 50 of navigationsystem 20. ECU 26 may comprise one or more programmable microprocessorsor microcontrollers or may comprise one or more ASICs. ECU 26 mayinclude a central processing unit (CPU) and an input/output (I/O)interface through which ECU 26 may receive a plurality of input signalsincluding signals generated by ablation generator 18, electrodes 42, 44on catheter 16, patch electrodes 48 of system 20, and input controlsystem 56 of RCGS 22 and generate a plurality of output signalsincluding those used to control and/or provide data to catheter 16,ablation generator 18, display 24, switch 50 of system 20, andmanipulator assembly 60 of RCGS 22.

In accordance with the present teachings, ECU 26 may be configured withprogramming instructions from a computer program (i.e., software) toimplement a method, such as a closed-loop feedback method, forcontrolling delivery of ablation energy from catheter 16 to tissue 12 inbody 14. The program may be stored in a local memory associated with ECU26, a remote memory accessible by ECU 26 over a telecommunicationsnetwork (e.g., on a file server) or on a portable storage medium such asa compact disc or on other types of computer readable storage mediums.Referring to FIG. 3, the method may begin with the steps 68, 70 ofreceiving one or more measurement signals from catheter 16 anddetermining, responsive to the measurement signal, a value for acharacteristic associated with the delivery of ablation energy to tissue12.

In accordance with one embodiment of the invention, the characteristicmay comprise a degree of contact between an ablation delivery element,such as electrode 42, on catheter 16 and tissue 12 and/or an amount offorce applied by the ablation delivery element to tissue 12. Themeasurement signal may be generated by one or more tactile or forcecontact sensors on catheter 16 configured to detect a force applied toelectrode 42 resulting from contact by electrode 42 with tissue 12 inorder to assess the degree of mechanical coupling between catheter 16and tissue 12. The contact sensors may generate signals indicative of achange in resistance, voltage, capacitance, impedance or a combinationthereof. The sensors may comprise, for example, capacitance sensors thatgenerate a signal indicative of a change in capacitance resulting fromapplication of a force. The sensors may also comprise piezoelectricsensors that include a piezoelectric material (in the form of a wire,film or tubes, for example) and generate a signal indicative of a changein voltage resulting from placing the piezoelectric material understress. The sensors may also comprise pressure sensitive conductivecomposite (PSCC) sensors (including, but not limited to, quantumtunneling conductive composite (QTC) sensors) in which the electricalresistance of the composite varies inversely in proportion to thepressure that is applied to the composite. Such sensors generate asignal indicative of a change in resistance or conductivity in thecomposite resulting from application of force. Additional information onexemplary sensor embodiments usable with the invention may be found inU.S. Patent Application Publication No. 2011/0022045, U.S. PatentApplication Publication No. 2008/0161796, U.S. Patent ApplicationPublication No. 2008/0015568, U.S. Patent Application Publication No.2007/0123764 and U.S. Patent Application Publication No. 2007/0100332,the entire disclosures of which are incorporated herein by reference.

The sensors may also comprise a pair of optically interactive elementssuch as an optically interactive surface and one or more optic fibersconfigured to emit and receive light energy from the electromagneticspectrum. The optically interactive surface has a known positionrelative to the ablation delivery element such that a change inposition, configuration and/or orientation of the surface causes achange in the plane of reflection and a change in a characteristic oflight (e.g., intensity, wavelength, phase, spectrum, speed, opticalpath, interference, transmission, absorption, reflection, refraction,diffraction, polarization and scattering) indicative of a force appliedto the ablation delivery element by, for example, contact with tissue12. The surface may be comprised of any material capable of reflectingor refracting light including, for example, polished metals. Refractivemedia (e.g., a lens or filter) or mediums (air, gel, or liquid includingthose dispersed or suspended in a solid or solid particulate) may beemployed with surface. Additional information on exemplary opticalsensing assemblies usable with the invention may be found in U.S. PatentApplication Publication No. 2008/0249522, U.S. Patent ApplicationPublication No. 2008/0275428 and International (PCT) Patent ApplicationPublication No. WO 2010/078453, the entire disclosures of which areincorporated herein by reference.

In accordance with another embodiment, the degree of contact may bedetermined by determining the degree of electrical coupling betweencatheter 16 and tissue 12 using measurement signals generated by one ormore of electrodes 40, 42 and indicative of impedance between theablation delivery element and tissue 12. As discussed in U.S. PatentApplication Publication No. 2010/0228247, U.S. Patent ApplicationPublication No. 2009/0163904, and U.S. Patent Application PublicationNo. 2010/0168735, the entire disclosures of which are incorporatedherein by reference, ECU 26 may enable generation of an excitationsignal from a signal source (not shown) across a path from an electrode42, 44 on catheter 16 to a return electrode located, for example, oncatheter 16 or body 14. This signal induces a response signal along apath from the catheter electrode to another return electrode on catheter16 or body 14 that is dependent on the complex impedance at thecatheter/tissue interface. Conventional circuits may be used to resolvethis signal into component parts for the complex impedance at thecatheter/tissue interface allowing ECU 26 to determine values for one ormore components of a complex impedance between the ablation deliveryelement and tissue 12. These components may including a resistancebetween the element and tissue 12, a reactance between the element andtissue 12, an impedance magnitude between the element and tissue 12 andan impedance phase angle between the element and tissue 12. ECU 26 mayfurther compute a coupling index responsive to these components andpossibly other measurements that are indicative of the degree of contactbetween the ablation delivery element and tissue 12.

In accordance with another embodiment, the characteristic associatedwith the delivery of ablation energy determined by ECU 26 may comprise alevel of conductivity in tissue 12, change in amplitude of anelectrogram, or another indicator of the quality of the lesion createdby the delivery of ablation energy to tissue 12. The measurement signalsuse to determine this characteristic may include temperaturemeasurements at the distal end 40 of catheter 16 using conventionaltemperature sensors on catheter 16 and/or impedance measurements usingelectrodes 42, 44. ECU 26 may use these signals alone or in combinationwith other parameters accessible through ablation generator 18 includingthe time duration of delivery of ablation energy and power levels.

In accordance with yet another embodiment, the characteristic associatedwith the delivery of ablation energy determined by ECU 26 may comprisean identity of a cardiac rhythm. The delivery of ablation energy has thepotential to cause ventricular tachycardia or other arrhythmias. ECU 26may identify the cardiac rhythm using electrogram readings obtained frommeasurement signals generated by electrodes 42, 44.

In accordance with yet another embodiment, the characteristic associatedwith the delivery of ablation energy determined by ECU 26 may comprisethe presence or amount of particulates in the bloodstream. In someinstances, ablation may result in dislodgement of particles that may becarried by the bloodstream to the brain and increase the risk of astroke. The measurement signals used to determine this characteristicmay include signals indicative of blood flow velocity in the bloodvessels such as the basal cerebral arteries. These signals may begenerated by ECU 26 or another device for processing images of the brainduring an ablation procedure such as the intraoperative transcranialDoppler monitoring system offered for sale by Natus Medical, Inc. underthe trademark Sonara.

In accordance with yet another embodiment, the characteristic associatedwith the delivery of ablation energy determined by ECU 26 may comprisethe presence or amount of micro-bubbles in the bloodstream. In someinstances, ablation may result in overheating of the blood and thegeneration of micro bubbles in the blood. The measurement signals usedto determine this characteristic may include signals indicative of thepresence or amount of micro bubbles in the bloodstream. These signalsmay be generated by ECU 26 or another device for processing images toidentify the presence or amount of micro bubbles in the blood. Theimages may comprise intracardiac echocardiography (ICE) images generatedby an ICE catheter.

In accordance with yet another embodiment, the characteristic associatedwith the delivery of ablation energy determined by ECU 26 may comprise adegree or type of motion of the body of a patient or a part of the body.For example and in some instances, cardiac ablation may impact the vagusand/or phrenic nerves which pass near the heart. Because the vagus andphrenic nerve affects the diaphragm and breathing, damage to one or bothof the nerves can result in vibration of the diaphragm that may besensed by a motion sensor and used to control further delivery ofablation therapy. As an alternative to a dedicated motion sensor,information regarding movement of the diaphragm or other respiratorymotion indicative of nerve palsy can be obtained from signals indicativeof such motion generated in the system offered for sale under thetrademark EnSite™ NavX™ by St. Jude Medical referenced hereinabove. Asanother example and in some instances, during renal ablation the renalartery may spasm and clamp down onto the ablation catheter. This spasmor motion may be sensed by a motion sensor (e.g., near the stomach) andused to control further delivery of ablation therapy.

The method may continue with the step 72 of generating one or morecontrol signals in response to the determined characteristic. Inaccordance with one aspect of the invention, a control signal may beused to control an amount of ablation energy delivered from the ablationdelivery element to tissue 12. The control signals may be provided toradio frequency generator 48 to control the ablation signal provided toablation catheter 16.

Referring now to FIG. 4, one embodiment of a control function that maybe implemented in ECU 26 to control the amount of energy delivered bycatheter 16 in response to a determined value of a characteristic isillustrated. In the illustrated embodiment, the characteristic comprisesthe degree of contact between the ablation delivery element and tissue12 which may be determined as described hereinabove. It should beunderstood, however, that similar functions may be used for otherdetermined characteristics. The function may define minimum and maximumthreshold values for the characteristic. Where the characteristiccomprises the degree of contact between the ablation delivery elementand tissue 12, the minimum threshold may represent insufficient contactor a lack of contact with tissue 12 while the maximum threshold mayrepresent a level of contact that risks perforation of the tissue and,in the case of cardiac tissue, cardiac tamponade. In accordance with oneaspect of the invention, the amount of energy varies in response to thedetermined value of the characteristic when the determined value of thecharacteristic meets a predetermined condition relative to at least oneof the threshold values. Systems have been proposed previously thatwould control ablation energy in response to the degree of contact, butthese systems rely on a relatively simple on/off control methodology inwhich ablation energy is delivered, at a presumably constant level, whenthe degree of contact is between minimum and maximum threshold valuesand delivery of energy ceases when the degree of contact is less thanthe minimum threshold value or greater than the maximum threshold value.These prior systems do not allow for optimization of energy delivery totissue 12 and can result in undesirable amounts of energy applied whenthe degree of contact is near the threshold values as well asundesirable oscillation between disparate energy levels. As shown inFIG. 4, the inventor herein proposes a control function wherein theamount of energy delivered to tissue 12 continuously varies in responseto the value of the degree of contact (or another characteristic) whenthe degree of contact is greater than or equal to the minimum thresholdvalue or less than or equal to the maximum threshold value. By varyingor titrating the amount of energy, finer, optimized control of ablationenergy is achieved. In accordance with the illustrated embodiment, theamount of energy assumes a maximum value when the degree of contact isbetween the minimum and maximum threshold values and, in particular,when the degree of contact is midway between the minimum and maximumthreshold values. Further, for values of the degree of contact includingand between the minimum and maximum threshold values, the amount ofenergy assumes a minimum value when the degree of contact is at one orboth of the threshold values and increase as the difference between thevalue of the degree of contact and either of the threshold valuesincreases until reaching the maximum value at the midpoint of the twothreshold values. In the illustrated embodiment, the amount of energyalso varies in response to the degree of contact when the value of thedegree of contact is less than the minimum threshold value or greaterthan the maximum threshold value. The amount of energy is higher,however, when the degree of contact is between the minimum and maximumthreshold values than when the degree of contact is not between thethreshold values (i.e., lower than the minimum threshold value or higherthan the maximum threshold value). Further, the amount of energydecreases moving away from the threshold values when the degree ofcontact is not between the threshold values.

Although in the illustrated embodiment, the amount of energy varies onboth sides of a threshold value, it should be understood that in analternate embodiment the amount of energy may vary only when the degreeof contact is between the threshold values and energy delivery may ceasewhen the degree of contact is outside of the threshold values therebyallowing for optimized control of energy delivery only when contact isdeemed sufficient to support delivery of ablation energy. Further,although the illustrated embodiment implements a Gaussian function todefines the amount of energy delivered to tissue 12, it should beunderstood that other control functions could be implemented that resultin continuous variation of the amount of energy delivered to tissue 12in response to the value of the degree of contact (or anothercharacteristic). For example, a Sigmoid function could alternatively beemployed where the amount of ablation energy delivered to tissue 12constantly increases or decreases between (and even outside of) thethresholds.

In accordance with one embodiment of the invention, the control functionimplemented by ECU 26 is established by the clinician using an inputdevice such as a graphical user interface on display system 24 or inputsystem 58 of RCGS 22. Using the input device, the clinician may selectfrom among a plurality of predetermined control functions stored in amemory associated with ECU 26. Alternatively, the clinician may create acontrol function by, for example, setting values for the amount ofenergy to be delivered by ablation catheter 16 in response to one ormore values for the characteristic. For example, the clinician mayestablish energy amounts at the minimum and maximum threshold values forthe characteristic and a midpoint between those threshold values. Theamount of energy for other values of the characteristic not establishedby the clinician may be determined by ECU 26 using conventionalinterpolation methods (e.g. by computing a spline through theestablished values). Alternatively, the clinician may establish aconstant amount of energy over one or more ranges of values for thecharacteristic. Referring to FIG. 5, for example, a graphical userinterface 78 may display a bar 80 or another symbol representing theamount of energy to be delivered by ablation catheter 16 in response toa representative value of a characteristic such as the degree ofcontact. Using an input device (e.g., a mouse, touch screen display,etc.), the clinician may directly touch the applicable portion of thescreen with his or her finger and/or move a cursor 82 to grab onto thetop line of a bar 80 and raise or lower the bar 80 to thereby adjust theenergy output for one or more values of a characteristic. In addition tothe graphical representation, the amount of energy may be shown as adecimal percentage of total output. The amount of energy for othervalues of the characteristic not established by the clinician may againbe determined by ECU 26 using conventional interpolation methods (e.g.by computing a smooth spline through the levels set by the clinician).The interface may allow the clinician to increase the number of bars 80(or other symbols) through which the clinician may enter amounts ofenergy for additional values of the characteristic and additional detailto the control function. Further, although the illustrated embodimentshows a smooth function, the interface may allow the clinician toestablish a hard transition either by modifying a threshold or allowingdiscontinuities in the control function. Accordingly, prior toimplementing step 68, ECU 26 may be configured to perform the step ofestablishing a control function correlating values for thecharacteristic and amounts of energy to be delivered by catheter 16 totissue 12. In one embodiment, this step may include the substeps ofidentifying a plurality of control functions correlating values for thecharacteristic and amounts of energy to be delivered by catheter 16 totissue 12 and selecting one control function from the plurality ofcontrol functions in response to an input signal generated through auser interface. In another embodiment, this step may include thesubsteps of receiving one or more input signals through a userinterface, each of the input signals setting an amount of energy for aselected value of the characteristic, and determining an amount ofenergy for unselected values of the characteristic (through, for exampleinterpolation).

Referring again to FIG. 3, in accordance with another aspect of theinvention, one or more control signals may also be provided tomanipulator assembly 60 of RCGS 22 to control the position of catheter16 relative to tissue 12 (e.g., to move catheter 16 nearer to tissue 12or further away from tissue 12). In one embodiment, step 72 may includethe substeps 74, 76 of comparing the determined value of thecharacteristic to a predetermined value and forming the control signalif the determined value of the characteristic meets a predeterminedcondition relative to the predetermined value. For example, if thecharacteristic is a degree of contact and the determined value indicatesthat the ablation delivery element on catheter 16 is further away fromtissue 12 than desired as indicated by the predetermined value, acontrol signal may be provided by ECU 26 to manipulator assembly 60 anddrive assembly 62 to move the ablation delivery element and increase thedegree of contact with tissue 12. Similarly, if the determined valueindicates that the ablation delivery element on catheter 16 has a degreeof contact with tissue 12 that is greater than desired as indicated bythe predetermined value, a control signal may be provided by ECU 26 tomanipulator assembly 60 and drive assembly 62 to move the ablationdelivery element and reduce the degree of contact with tissue 12. Thesesteps and substeps can be performed repeatedly such that drive assembly62 maintains substantially the same degree of contact between theablation delivery element and tissue 12 over time in response to controlsignals generated by ECU 26.

A system 10 and method for controlling delivery of ablation energy froman ablation catheter 16 to tissue 12 in a body 14 in accordance with thepresent teachings is advantageous because the system and method enablefiner control of the delivery of ablation energy to tissue in responseto changes in various characteristics associated with the delivery ofablation energy to the tissue. As a result, delivery of ablation energyinto the blood is minimized to reduce the risks of thrombus formation.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the scope of this invention. All directional references (e.g.,upper, lower, upward, downward, left, right, leftward, rightward, top,bottom, above, below, vertical, horizontal, clockwise andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not as limiting. Changes in detail or structure may be made withoutdeparting from the invention as defined in the appended claims.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialsdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as explicitly set forth hereinsupersedes any conflicting material incorporated herein by reference.Any material, or portion thereof, that is said to be incorporated byreference herein, but which conflicts with existing definitions,statements, or other disclosure material set forth herein will only beincorporated to the extent that no conflict arises between thatincorporated material and the existing disclosure material.

What is claimed is:
 1. A system for controlling delivery of ablationenergy from an ablation catheter to tissue in a body, comprising: anelectronic control unit configured to: determine, responsive to ameasurement signal from the ablation catheter, a value for acharacteristic associated with the delivery of ablation energy to thetissue; and generate a first control signal, responsive to thedetermined value of the characteristic, to control an amount of energydelivered from the ablation delivery element to the tissue; wherein theamount of energy varies in response to the determined value of thecharacteristic when the determined value of the characteristic meets apredetermined condition relative to a first threshold value for thecharacteristic.
 2. The system of claim 1 wherein the characteristiccomprises a degree of contact between an ablation delivery element onthe ablation catheter and the tissue.
 3. The system of claim 1 whereinthe characteristic comprises a level of conductivity in the tissue. 4.The system of claim 1 wherein the characteristic comprises an identityof a cardiac rhythm.
 5. The system of claim 1 wherein the measurementsignal is indicative of a temperature at a distal end of the ablationcatheter.
 6. The system of claim 1 wherein the measurement signal isindicative of an impedance between an ablation delivery element on theablation catheter and the tissue.
 7. The system of claim 1 wherein themeasurement signal is indicative of an electrogram amplitude.
 8. Thesystem of claim 1 wherein the amount of energy also varies in responseto the determined value of the characteristic when the determined valueof the characteristic does not meet the predetermined condition relativeto the first threshold value.
 9. The system of claim 1 wherein theamount of energy assumes a maximum value when the determined value ofthe characteristic is between the first threshold value and a secondthreshold value.
 10. The system of claim 9 wherein, for values of thecharacteristic including and between the first and second thresholdvalues, the amount of energy assumes a minimum value when the determinedvalue of the characteristic is at one of the first and second thresholdvalues.
 11. The system of claim 1 wherein the first threshold value isindicative of a relatively low value of the characteristic and a secondthreshold value is indicative of a relatively high value of thecharacteristic and the amount of energy is higher when the determinedvalue of the characteristic is between the first and second thresholdvalues than when the determined value of the characteristic is notbetween the first and second threshold values.
 12. The system of claim1, further comprising: a radio frequency generator configured togenerate an ablation signal responsive to the first control signal, theablation signal controlling the amount of energy delivered from anablation delivery element on the ablation catheter to the tissue. 13.The system of claim 1, further comprising a remote catheter guidancesystem configured to control movement of said ablation catheter whereinsaid electronic control unit is further configured to generate a secondcontrol signal, responsive to the determined value of thecharacteristic, and provide said second control signal to said remotecatheter guidance system.
 14. The system of claim 1 wherein saidelectronic control unit is further configured to establish a controlfunction correlating desired amounts of energy to be delivered from theablation delivery element to the tissue with values for thecharacteristic.
 15. The system of claim 14 wherein said electroniccontrol unit is further configured, in establishing said controlfunction, to: receive a plurality of input signals, each of said inputsignals indicative of an amount of energy to be delivered by theablation delivery element to the tissue for one of a plurality ofselected value of said characteristic; and, determine an amount ofenergy for a plurality of unselected values of said characteristic. 16.An article of manufacture, comprising: a computer readable storagemedium having a computer program encoded thereon that when executed byan electronic control unit controls delivery of ablation energy from anablation catheter to tissue in a body, the computer program includingcode for: determining, responsive to a measurement signal from theablation catheter, a value for a characteristic associated with thedelivery of ablation energy to the tissue; and generating a firstcontrol signal, responsive to the determined value of thecharacteristic, to control an amount of energy delivered from theablation delivery element to the tissue; wherein the amount of energyvaries in response to the determined value of the characteristic whenthe determined value of the characteristic meets a predeterminedcondition relative to a first threshold value for the characteristic.17. The article of manufacture of claim 16 wherein the amount of energyassumes a maximum value when the determined value of the characteristicis between the first threshold value and a second threshold value. 18.The article of manufacture of claim 16, wherein the computer programfurther includes code for generating a second control signal, responsiveto the determined value of the characteristic, to control a remotecatheter guidance system and movement of a distal end of the ablationcatheter.
 19. The article of manufacture of claim 16 wherein thecomputer program further includes code for establishing a controlfunction correlating desired amounts of energy to be delivered from theablation delivery element to the tissue with values for thecharacteristic.
 20. The article of manufacture of claim 19 wherein thecode for establishing the control function includes code for: receivinga plurality of input signals, each of said input signals indicative ofan amount of energy to be delivered by the ablation delivery element tothe tissue for one of a plurality of selected value of saidcharacteristic; and, determining an amount of energy for a plurality ofunselected values of said characteristic.